The goals of this proposal are to detect, to quantitate and to separate cell or nuclei containing estrogen receptors by a flow flourometer. The cells or nuclei will be derived from three sources: nine established human breast cell lines, tissue from immature rat uteri, and human breast cancer tissue. A prepared fluorescein labeled estradiol which competes with 3H-estradiol for receptor binding sites, will be incubated with cellular or nuclear suspension to form a steroid-receptor complex. This complex in the cell or nuclei will then be analyzed and or separated by the degree of fluorescence which would be directly related to the receptor concentration. This procedure would quantitate the receptor concentration on a cellular basis (% of cells that are estrogen receptor positive) rather than a cytosol protein concentration parameter. Receptor concentration determined by standard radioactive procedures would be correlated with all fluorescence determinations in order to standardize the fluorescent data and both parameters would be related to responsiveness of patients treated with hormonal therapy. Futhermore, the cell cycle kinetics will be determined by DNA histograms using the flow fluorometer and this data compared to estrogen receptor concentrations. Subsequent to sorting the estrogen receptor positive cells or nuclei (high fluorescence) from the estrogen receptor negative cells or nuclei (low fluorescence), these two populations will be studied by electron microscopy cell culture techniques and DNA content. The fluorescence assay described above has many advantages over the standard 3H-steriod procedures presently used for steroid receptor analysis. It is fast, sensitive and able to use small amounts of tissue. The principal investigator also believes that it would be more diagnostic of tumor type and therefore, serve as a better indicator for hormone responsiveness in breast cancer patients.